The synthesis of molecular composites, that is composites formed from molecules comprised of stiff or rigid molecular segments, interspersed with flexible segments, has attracted considerable attention in the recent past. In this regard, the reinforcing fibers contained in ordinary composites suffer from the fact that they have a tendency to separate from the material in which they are incorporated, thereby impairing the composites' physical properties. In addition, a substantially homogeneous dispersion of fibers in a composite matrix is frequently difficult to achieve. Partly as a consequence of the foregoing, therefore, there have been many attempts to develop so-called "molecular composites," using techniques designed to overcome the disadvantages described.
Unfortunately, however, molecular composites as they are presently understood might be more accurately described as an objective desirable to achieve, rather than one already accomplished. In this regard, true molecular composites, i.e., composites in which molecular segments are homogeneously distributed in flexible matrix molecular segments are difficult to prepare. This is in consequence of the fact that such rigid segments or "rods" tend to aggregate after reaching a critical concentration. Such aggregates are, in fact, to be expected, being predicted by Flory's theory, Polymer 28, 21-30 (1987). In any event, the heterogeneity commonly experienced significantly impairs the structural reinforcement that molecular composites are designed to provide.
A further problem heretofore experienced with molecular composites arises from the fact that due to their tendency to form domains of concentrated rigid-rod structures, the composites tend to resist thermal processing, making their fabrication into commercial products difficult to achieve.
In the past, a number of techniques have been proposed in order to overcome the difficulties described. For example, it has been suggested that molecules with rods less rigid be used; that matrix molecules having a more flexible character be employed, or both. Where these expedients are resorted to, however, the products obtained frequently prove to have less than desirable physical properties.
A different approach suggested involves fixing the position of the rigid-rod molecules within the matrix molecules, for example, by first dissolving the rigid-rod-containing molecules in a solution of a thermosetting material intended to form the matrix. The mixture is then subjected to curing conditions, thereby causing the dispersed rods to be immobilized within the resulting crosslinked matrix. While homogeneity is achieved, however, the resulting products possess the physical characteristics of the thermoset resin employed, and for that reason, the composites are not always suitable for the end-use required of them.
Still another technique proposed involves the formation of the rigid rods only after a homogeneous mixture of the rods and matrix has been achieved. In this method, coil-like molecules are combined with the matrix material, and following mixing of the two, the mixture is processed to form the desired product. Thereafter, the coil-like molecules are chemically altered to provide the rigid-rods necessary to reinforce the composite. Such in-situ methods, however, present certain problems relating to molecular dynamics, and are to that extent undesirable.
In an effort to prepare useful molecular composites, and at the same time overcome the problems described, attempts have also been made to synthesize molecules that combine both rigid-rod segments and flexible matrix segments within the same molecule. In connection with this approach, resort has been had to the preparation of block copolymers characterized by possessing rigid-rod reinforcing blocks in combination with flexible matrix blocks. In addition, flexible matrix-forming polymer side chains have been grafted onto polymer backbones that include rigid-rod reinforcing segments. Even with these systems, however, one is still confronted with molecular mobility sufficient to cause phase separation of the composites, resulting in the formation of discrete domains of concentrated rigid-rod segments, and portions of the composites in which the matrix segments predominate. Such separation is due to the basic incompatibility of these segments, and thus far undesirable separations of the type described have been difficult to avoid.